The demands for positional accuracy of precision machining tools have become more stringent in recent years. This has led to an increased demand for position measuring instruments, whether they be length or angle measurement devices having resolutions in the submicrometer range. Measuring instruments that operate interferentially, known as grating interferometers, have been used.
In general, such machines provide a measurement value by interfering at least two beams of light diffracted at a scale graduation and evaluating the phase difference. Equation (1) below defines the phase displacement .OMEGA. of a wave of the order n diffracted at a scale grating for a displacement x of the scale having a graduation period c: EQU .OMEGA.=(n*2 .pi.*x)/c (1),
where
* denotes the operation of multiplication. From equation (1) it can be seen that the wavelength of the light source has no affect on the phase displacement. Thus the measurement signal is derived solely from the graduation of the scale grating and is thus only as accurate as the graduation.
The scale gratings with phase graduations used in interferometers have been produced by the processes of lithography or holography. An advantage of lithographically produced scale gratings is that gradations with lengths of over 1 meter can be produced, while in holographically produced scale gratings because of the high demands made on the imaging optics, the length is limited to less than 250 mm.
Such gratings have profiled phase gratings on their surface and thus are intrinsically vulnerable to becoming soiled especially when such measuring instruments are used in dirty environments, such a machine tooling. An example of such a grating is found in U.S. Pat. No. 4,776,701 (Petrigrew) herein incorporated by reference.